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Drilling Small, Deep Holes with Precision EDM

Quality production results for aircraft, medical applications

By Jim LorinczSenior Editor

Electrical Discharge Machining (EDM) has been removing metal by spark erosion for more than half a century—with sinker (ram/Elox-) style EDMing for molds and wire EDM for precision parts cutting, especially dies. A third type, EDM drilling, has emerged for precision holemaking in the smallest sizes, going beyond its use initially as an EDM “popper” for starting holes in hard metals.

Today, there is no end in sight for precision machining applications like small holemaking for which EDM drilling in combination with other processes is being used. Originally an alternative to machining processes like turning and milling, EDM is often used in combination with other advanced machining processes. It has flourished as technology embraced advances in digital generators for more precise spark control and software that draws upon a legacy of machining data for more efficient production.“The hot button for EDM drilling right now is shaped cooling holes for new engines in the aircraft industry,” said Bob Ianitelli, president, Belmont Equipment and Technologies (Madison Heights, MI). “I’ve also seen an increase in requests for very small holes in the 0.003–0.004" [0.08–0.10-mm] diameter range in production applications for new fuel-saving aircraft engine designs that require cooling holes for fan blades that are increasingly more complex. There isn’t one shape that everyone is using, some are trapezoidal looking shapes. Some are double angle 'V' shapes compared with conventional tapered shapes,” said Ianitelli.

“Right now what’s up for discussion is how to create the different shapes,” said Ianitelli. “Some are done on sinkers, lasers, and EDM drills. Which shapes are the most efficient is still up for discussion, as it is application based.” Selecting the right process for the application is very much Belmont Equipment’s approach to designing a machining solution matched to its customer’s application. “We talk about the application first and are able to partner to the level that our customer wants. We can supply just the machine, or provide a total engineered turnkey process with the part, and proven on our shop floor before being shipped to the customer.”

Where two methods of EDM are used, an EDM drill in conjunction with a sinker or high-speed mill, a palletizing system (whether it’s from EROWA, 3R, or Hirschmann) allows moving parts from one machine to the other without any mismatch. “Palletizing is our recommended way of doing it if you’re using a two EDM machine process,” said Ianitelli.

Not only do holes vary in size for aircraft engines, and automotive injectors, but medical implants continue to challenge the limits of small-hole machining with EDM. “If you look at drilling a hole the size of 0.004", it’s a really difficult hole to EDM. You’re dealing with something the size of a hair that has to be loaded into a machine. Sometimes just the heat from your fingers can distort what you’re handling. We have the burning technology and are able to drill those in one-offs, but production is a different story, especially for fuel injector applications.”

Developments in small-hole EDM drilling are shrouded in mystery to a certain degree about the exact machining techniques that are being used. “Even at trade shows,” said Ianitelli, “Nobody really shows how they are putting shapes in with their EDM drilling processes.”

On another note, Ianitelli said that there is a definite increased need in the market for large sinker EDM machines. “We’re seeing a big uptick in larger EDM sinkers for making the smaller cuts that are going into large tools. Where in the past, the mold for an entire instrument panel or fascia would have been EDMed with one large electrode, now our large two-headed sinkers are EDMing smaller cuts for the clips that are holding them on,” said Ianitelli.

Drilling Smaller, Deeper, and More Precise Holes

EDM holemaking technology is being driven by demand for smaller, deeper and more precise holes produced as fast and as consistently as possible. “What essentially started with conventional EDM hole-popper machines has evolved into advanced EDM drilling systems that further enhance the capabilities of their predecessors in terms of hole size, depth, surface finish and accuracy, as well as in process speed, reliability and simplicity,” said Ken Baeszler, product manager at GF AgieCharmilles (Lincolnshire, IL).“In the past, it was a truly significant accomplishment to drill a 0.006" [0.150-mm] diameter hole 0.125" [3-mm] deep in hardened steel using a 0.004"-diameter electrode on an EDM hole-popper machine. Today’s technology makes it possible to drill the same hole upwards of twice as deep or drill one that is 0.040" in diameter 6" deep and varying in size and straightness no more than 0.001" [0.025 mm] from its entrance to exit. Plus, being able to do it using only one electrode and little, if any, operator intervention,” said Baeszler.

Present EDM drilling systems are fundamentally a merging of EDM hole-popper technology with that of die-sinker EDMs. At the heart of these precision machines is the latest digital generator technology adapted from the die-sinker side of the family. “These state-of-the-art digital generators are much more efficient with higher output that delivers speed, consistency and precision. And they consume less energy,” said Baeszler.

GF AgieCharmilles’ Intelligent Power Generation (IPG) generator provides precise spark control for the EDM drilling machines with different parameter settings for the various stages of the drilling process and according to where the electrode is in the hole. “It is this functionality that also helps reduce electrode wear, which, in turn, allows for deeper drilling, increased speed and accuracy and higher surface-finish quality,” said Baeszler.

Spark control is accomplished via software in the digital generator that modifies or adjusts spark behavior as the electrode enters a hole and when it exits—a function often referred to as entrance and breakthrough protection. “These protection functions, such as our Intelligent Breakthrough Detection, prevent instability during hole exiting, shorten actual electrode exiting time, eliminate back-wall striking and help maintain hole diameter size at the point of exit. The function also calculates the necessary electrode retract length so that it can be safely moved to the next hole, as well as how much of the electrode is left and if that remaining amount is enough to do the next hole or if an automatic electrode change should be initiated,” said Baeszler.

These capabilities are what allow modern EDM drilling systems to accurately, efficiently and automatically process those workpieces involving multihole and/or multielectrode size operations. In the case of a conventional hole popper, the operator—a highly skilled one is required—would have to perform all of these functions. “For increased EDM drilling productivity, the goal is to generate as many holes as possible or drill as deep as required using the least amount of electrodes. The more a generator can help reduce electrode wear, the better. Plus, less wear on electrodes allows them to cut faster and more accurately by eliminating ‘bullet nosing’ of their tips,” said Baeszler.

Lab Testing Confirms Small-Hole EDM Drilling Results

To illustrate, a conventional popper system using a 0.040" diameter electrode to drill through about 3.937" (100 mm) of hardened steel, would typically experience 100–150% of electrode wear. However, in lab testing, GF AgieCharmilles accomplished the same task on its new Drill 300B, a three-axis high-speed EDM drilling system with IPG Generator technology, and experienced an electrode-tip wear rate of only 45%.“Also during testing, we used a 0.040"-diameter electrode and burned a hole 1.181" [30-mm] deep in a carbide test part in just 4.5 minutes at a miniscule electrode-tip wear rate of less than 50%. Without digital generator technology, the same hole would have taken over 15 minutes to complete with upwards of 500% electrode wear rate. Such excessive wear, in some cases, can dictate the use of multiple electrodes as well as reduce overall accuracy of the hole. These test hole sizes and depths that we achieved could never be produced using mechanical methods,” said Baeszler.

“When it comes to hole accuracy, the more an electrode wears, the greater the advent of uneven wear and the higher the tendency for the electrode to ‘walk’ off center, resulting in crooked and angled holes as opposed to straight ones,” said Baeszler. “Because they help reduce wear, digital generators thus prevent such a scenario and contribute to producing straight holes and ones that can be held closer to size from top to bottom. For instance, a 5.905" [150-mm] deep hole that we drilled with a 0.04" electrode varied in size from top to bottom no more the 0.001" [0.025 mm].”

Besides increasing electrode life, digital generator technology also provides control over heat-affected zones, resulting in surface finishes comparable to those typically attained using a wire EDM. Such surface finishes are possible on conventional hole-popper machines, but achieving them would require an extremely skilled operator and a considerable amount of time. With the latest systems, on the other hand, the process is consistent, much quicker and accomplished automatically.

Because they incorporate more of the design qualities of their sinker-EDM parent machine technology, today’s advanced EDM drilling systems, such as the Drill 300B, are built for continuous untended, lights-out operations. And because they provide very accurate positioning along with high-quality surface finishes, an increasing number of shops use the machines for generating finished holes, those that are actually part features, as opposed to an EDM start hole and that won’t require any secondary finishing operations. In addition, with higher positioning accuracy capabilities, an EDM drill machine’s use has further expanded into multiple-hole part applications, especially when the holes are different diameters and must be precisely located on the parts.

EDM Drilling Small, Tight Tolerance Holes

Fast hole EDM drilling machines can be used to efficiently manufacture small, tight tolerance holes, for example, in medical parts that require the insertion of suture material and guide wires. Recent enhancements in this technology include controller and power supply upgrades. “Software and hardware upgrades to the controller and EDM power supply improve drilling speed and stability, while reducing electrode usage and improving metallurgical recast issues,” said Steve Bond, sales manager- EDM Products, Methods Machine Tools Inc. (Sudbury, MA).

“In addition, the RT6050-5AT is designed with an extended traveling bridge X-axis. Depending on application requirements, the machine can be equipped with either a fixed or tilting EDM drilling head, and the machine table is designed to accept various sizes of rotary tables.

“New power supply technologies enable drilling with less side-wear at the tip of the electrode. This results in more consistent hole depth and diameter, faster drilling speeds and more holes per electrode. In an effort to reduce cycle time, automatic electrode changers have also been upgraded to shorten electrode change time and optimize electrode usage, leading to fewer overall electrode changes,” said Bond.

New servo controls with advanced gap sensing improve drilling stability and reduce electrode flexing, especially when using longer, small diameter electrodes. In addition, new TRACE functions further the ability to optimize depth control by providing a graphical method to fine-tune depth sensing when drilling into small pockets.

Wire EDM Designed for Ease of Use, Low Cost of Operation

Advances in wire EDM technology focus on programming and software is designed for ease of use, as well as accuracy and quality of finish. Such is the case with two new large scale wire EDM machines from Makino Inc. (Auburn Hills, MI). The U3 and U6 wire EDMs feature new HyperCut technology and Hyper-i control that make is easy for even novice operators to program and produce the most complex parts in competitive cycle times and high degrees of accuracy and finish, regardless of the condition or application.

Both machines are scaled for large payloads and travels and cost-effective machining. The U3 will accommodate a maximum workpiece size of 770 × 590 × 220 mm with a payload of 1322 lb (600 kg). The U6 holds sizes up to 1000 × 800 × 400 mm with a maximum payload of 3307 lb (1500 kg). The U3 and U6 feature an innovative design in which the entire bed casting is used as the dielectric reservoir. This design approach reduces floor-space requirements over previous models and eliminates the need for additional external fluid tanks. ME

This article was first published in the November 2013 edition of Manufacturing Engineering magazine. Click here for PDF.